symvar: document a difference with SMT and symvar of symfuns

I think this is a bug in SMT for single input form: symvar(symfun), at
least our treatment is consistent with symvar(symfun, n).

Change the test to be our way, instead of xtest on the SMT approach.
Add symfun/argnames command to return the independent vars of a symfun

Fixes #175.  Or more accurately, means I don't care about #175 ;-)
Users should use "argnames(f)" instead of "f.vars" directly.